change account associated with Mail mailbox

When replying to, or creating new, messages from my wife's "On My Mac" mailbox, by default they will be sent from my mail account instead of her mail account. The Addressing setting "Send new messages from:" in preferences is set to "Account of selected mailbox". I am not sure when this stopped working, but I am pretty sure that before I started playing around to try to get this working again, the account associated with the mailbox was set to my wife's account but her replies and new messages would default to my account anyway. Since I have started playing around (including turning on and off the "Automatically detect and maintain account settings" preference) I have not been able to set the account associated with this mailbox to that of my wife. The only way I can see to access this setting is by selecting "Account Info" window, but changing the associated account in this window does not appear to make any difference or even to hold the changes. Other default mailboxes (her personal Inbox, Drafts, Sent and Trash) still work as expected. The mailbox in question is not a smart mailbox but uses rules to deliver her mail. However, the smart mailboxes also appear to have this problem.

Is there some solution to this problem?

thanks,

Paul

Mail v. 9.3

OS 10.11.6